Surplus-labor was not invented by the capitalists. Ever since human societies issued from the state of primitive communism, surplus-labor has always existed; and it is the method by which it is wrung from the immediate producers, which differentiates the different economic forms of society. Before man was able to produce in excess of his needs, one portion of society could not live upon the fruits of the toil of another portion. How could a man work gratuitously for others when his entire time was barely sufficient to procure him his own necessary means of existence? When, in consequence of human progress, labor had acquired such a degree of productiveness that an individual was enabled to produce more than what was strictly necessary for his needs, it became possible for some to subsist upon the toil of others and slavery could be established. That it was established by force is not doubtful; but it must be confessed that its establishment promoted human evolution. So long as the productiveness of labor, although sufficient to make surplus-labor possible, was not sufficient to render participation in directly useful labor compatible with other occupations or pursuits, the toilsome drudgery and exploitation of some was the necessary condition of the leisure of others, and, thereby, of the development of all. For, if none had had leisure, no progress could have been made in the sciences, the arts and all the branches of knowledge, the benefits of which we all enjoy in some degree. And the fact that the thinkers of antiquity and the greatest among them, Aristotle, excused slavery, is a proof that the mode of thought is determined by the exigencies of the economic organization of society. To reproach Aristotle, in particular, because he did not regard slavery and property as it is natural for us to regard them, is equivalent to reproaching him for not having applied the processes of our modern production to ancient industries. Slavery did not appear to lack a rational foundation, and did not begin to disappear until the external conditions were profoundly transformed and thus rendered another kind of labor and of surplus-labor more in harmony with the material requirements. Following upon the economic environment in which slavery was the rule there came then the economic environment in which serfdom predominated, and the latter, in its turn, has been superseded by the economic environment in which the wage-system has become the general rule. Each of these environments has had or has its own habits and modes of thought which may be in contradiction with ours, but which are the natural consequences of the modes of life in vogue in their respective eras.